Attenuation and termination circuit using impedance synthesis

ABSTRACT

The present invention discloses circuits for isolating and attenuating signals generated by a telephone network. In disclosed embodiments, a metering pulse signal is isolated from the terminals of the connecting device, and then attenuated with an impedance that is synthesized with a programmable digital signal processor. Embodiments also utilize the digital signal processor to synthesize a termination impedance for the connecting device. The termination impedance matches closely the characteristic impedance of the network, so as to minimize wave reflections and the like.

RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 09/560,581, filed on Apr. 28, 2000 now U.S. Pat. No. 7,660,408,herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to circuitry for terminating andattenuating electrical signals. More particularly, the present inventionrelates to circuitry for isolating and attenuating incompatible signalspresent on a telephone network, such as a metering pulse signal.

2. Present State of the Art

The primary function of a telephone network (commonly referred to as thepublic switched telephone network, or PSTN) is to transmit audiosignals. Accordingly, telephone networks have been optimized to carrysignals having frequencies and amplitudes that fall within the audiospectrum. However this has limited the ability to utilize the telephonenetwork as a transmission medium for other types of signals fallingwithin different frequency spectrums. Similarly, problems can arise whennon-traditional telephone equipment, such as modems, are connected tothe telephone.

For a device to connect to the telephone network, it must be able tointerpret the various signals that are present on the telephone network.In addition, the device must comply with various standards that areimposed by the telephone network. For instance, in the U.S., to connectto the PSTN, the central switching office places a direct currentvoltage of approximately −48 volts on the telephone line, and requiresthat a connecting device, such as a telephone or modem, draw little orno current when the device is not in use. This requirement is satisfiedby designing the connecting device to have a large, specified impedancewhen the device is not in use, referred to as the on hook state. Thislarge impedance effectively creates an open circuit which draws littlecurrent.

Another example of a PSTN mandated requirement is that when the deviceis being used (off-hook), the impedance of the device must closely matchthe characteristic impedance of the telephone network, typically 600ohms. This ensures that the signal being transmitted over the telephonenetwork is not reflected due to an impedance mismatch. Thus, aconnecting device must be capable of presenting different impedances tothe telephone network, depending on its connection state.

The design of a telephone network-connecting device becomes more complexwhen signals having frequencies and amplitudes that are different fromconventional audio signals are present on the telephone network. Onesuch signal is referred to as a “metering pulse signal.” While notpresent in the U.S. PSTN, in some countries, the telephone networkgenerates metering pulse signals that have a carrier frequency at orabove 12 kHz. The metering pulse signal permits entities such as smallbusiness to obtain instant billing information so that the cost of atelephone call can be identified. By instantly knowing the cost of atelephone call, the business can charge its customers appropriately.

The metering pulse signal, in addition to having a frequency that istypically higher than voice communications, has peak amplitudes on theorder or 20 to 40 volts. As such, the signal can be detrimental to theoperation of modems and other connecting devices. For example, thesignal can saturate the receive amplifier, corrupt the data, and canphysically damage electrical components of the connecting device. Inaddition, the metering pulse signal, like other signals present on thetelephone network, is subject to certain specifications. In particular,the metering pulse signal cannot be attenuated by more than a prescribedamount of about 3 dβ at the input terminals of the connecting device.

One solution to the problem presented by signals such as the meteringpulse signal is to use an external podule that contains an attenuationfilter. However, this approach is not entirely satisfactory because ofthe additional cost incurred in order to manufacture, stock anddistribute the podules. In addition, the customer is burdened with notonly additional equipment that must be connected to the modem, but alsowith the additional cost associated with purchasing the podules.

Thus, it would be an advancement in the art to provide the ability tolimit the detrimental effects of incompatible telephone network signals,such as metering pulse signals, on certain network connecting devices,such as modems. Moreover, it would be an advance in the art to provide aprotection scheme that doesn't require extra external equipment, such asan external filtering podule.

OBJECTS AND SUMMARY

In view of the foregoing and other problems in the prior art, it is anoverall object of one embodiment of the present invention to provide asystem and method that allows a connecting device to operate on atelephone network in the presence of an incompatible network signal, byelectrically isolating and attenuating the incompatible signal.

Another objective is to provide a system and method that accomplishessuch signal isolation and attenuation without using equipment, such asfilter podules.

It is a another object to provide a system and method that attenuates anincompatible signal present on a telephone network or othercommunication network by automatically generating and providing asynthesized impedance, thereby preventing the signal from interferingwith the operation of a connecting device, such as a modem.

Another objective is to isolate and attenuate the incompatible signal,such as a metering pulse signal, in a manner that does not interferewith the operation of the telephone network, or in a manner thatotherwise violates the operating characteristics of the network.

It is also an objective of embodiments of the present invention toautomatically synthesize a termination impedance that substantiallymatches the impedance of the telephone network, thereby preventing wavereflections and signal losses.

In summary, these and other objectives are obtained with embodiments ofthe present invention, that include a circuit that isolates a networkdevice from frequency-undesirable or frequency-incompatible signals thatmay be present in a telephone network, and that utilizes a synthesizedimpedance to attenuate the signal level of any undesirable networksignal. Because the signal is attenuated, it does not interfere with theoperation of the network connected device. Embodiments also provide theability to synthesize desired termination impedance, so that the networkconnected device matches the characteristic impedance of the network.

One example of the type incompatible signal addressed is the meteringpulse signal described above. As was noted, a metering pulse signal hasa carrier frequency on the order of 12 kHz or more, and has peakamplitude of 20 to 40 volts or more, which can be incompatible with andeven damaging to certain network connected devices, such as a modem.

The detrimental effects of this type of incompatible network signal areprevented when embodiments of the present invention are implemented innetwork connected devices. Presently preferred embodiments provide anelectrical path for the metering pulse signal (or other incompatiblesignal) to electrically isolate it from the terminals of the connectingdevice. For example, in one embodiment a resistive circuit that islocated such that the termination impedance of the connecting device issubstantially unaffected performs the isolation. Moreover, the approachensures that the metering pulse signal is not attenuated more than 3 dβacross the tip and ring terminals, less than an amount that wouldviolate the requirements of the PSTN.

Once the metering pulse signal has been electrically isolated, animpedance synthesis circuit automatically generates impedance. Theimpedance value is such that the metering pulse signal is attenuated bya predetermined amount at the end of the isolating resister away fromthe wire pair terminals, such as the previously mentioned tip and ringterminals. The impedance synthesis circuitry effectively performs thefunction of a notch filter, but does not require the use of physicalcircuit components typically used in filters, such as capacitors,inductors, and the like. Also, electrical components already existingwithin the modem or other network connected device can be used toperform the impedance synthesis function. This eliminates the need forany external, or additional parts, thereby reducing cost, manufacturingcomplexity, and operational complexity.

In addition, in preferred embodiments, the impedance synthesis circuitryis implemented so as to be much more precise than conventional filterpodules using discrete components, because the impedance synthesis isimplemented in a manner so as to preferably emulate the behavior ofideal circuit components by way of a digital signal processor. Theimpedance synthesis, in combination with the isolation resistor,effectively eliminates the harmful effects of the metering pulse signal(or similar signal) on connecting devices such as modems. In operation,the attenuation circuitry can cause the metering pulse signal toeffectively disappear at the input of the receive amplifier of a modem,or similar network connected device. As such, the metering pulse signalhas no detrimental effect on the operation of the device. Moreover, themetering pulse signal is not attenuated at the terminals of theconnecting device in violation of the specifications of the telephonenetwork.

Embodiments of the present invention also generate a desired terminationimpedance for the network connected device. Preferably, this synthesizedimpedance matches the characteristic impedance of the network, so as toavoid wave reflections, and thereby maintain compatibility with thenetwork.

Additional objects and advantages of the present invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by the practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instruments and combinations particularly pointed out inthe appended claims. These and other objects and features of the presentinvention will become more fully apparent from the following descriptionand appended claims, or may be learned by the practice of the inventionas set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand objects of the invention are obtained, a more particular descriptionof the invention briefly described above will be rendered by referenceto specific embodiments thereof which are illustrated in the appendeddrawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be consideredlimiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1 is an illustration of an exemplary system for implementing thesystems and methods of the present invention;

FIG. 2 is a block diagram of one presently preferred embodiment of thecircuitry for isolating, and attenuating undesirable signals present ona telephone network, and for providing a desired termination impedance;

FIG. 3 is a block diagram illustrating one embodiment of the impedancesynthesis circuitry present in the termination and attenuation circuitryof a connecting device; and

FIG. 4 is a detailed schematic illustrating additional details of oneembodiment of the impedance synthesis circuitry in the attenuationcircuitry and the termination circuitry of a connecting device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Devices that connect or interface with a telephone network are usuallydesigned to comply with standards specified by that telephone network.For example, the specifications may require that signals have certainfrequencies and amplitudes and/or may require that the connectingdevices present a specified termination impedance to the network. Adevice that does not comply with specified requirements may not operatewith the telephone network, and may have an adverse effect on theoperation of telephone network.

The design of a telephone or other connecting device is complicated bythe fact that many differences exist between the specifications oftelephone networks in different countries. In some instances, atelephone network may produce signals that are not present on othertelephone networks, resulting in connecting device incompatibility. Theability of a connecting device to function in more than one telephonenetwork is dependent on its ability to deal with such incompatiblesignals.

One example of a signal that is incompatible with certain connectingdevices, such as modems, is known as a metering pulse signal. Themetering pulse signal typically has a carrier frequency greater than 12kHz and has peak voltages on the order of 20 to 40 or more volts. Asdescribed previously, the signal can disrupt the communicationcapabilities of the connecting device. In the case of a modem, themetering pulse signal may cause the communication to end prematurely ormay introduce erroneous data into the communication. Additionally, thehigh voltages of the metering pulse signal can damage the modem'selectrical components, which are usually designed to operate withvoltages lower than 5 volts.

Embodiments of the present invention overcome the challenges presentedby the metering pulse, or similar types of incompatible signals. Also,presently preferred embodiments can be integrated or incorporated with aconnecting device, thereby eliminating the necessity of having externalfilters. Moreover, the number of additional circuit components isminimized by taking advantage of components and systems that may alreadybe present on a connecting device. The elimination of podules and theutilization of existing subsystems lowers the overall cost andcomplexity of the connecting device.

Preferred embodiments of the present invention utilize systems andmethods that introduce superior accuracy compared to solutions that usephysical circuit elements because the metering pulse is attenuated bysystems and methods that are comparable to ideal circuit elements. Also,preferred embodiments of the present invention can be easily adjusted toaccommodate the various frequencies which may be present on differenttelephone networks. It will be appreciated that while the presentinvention is described in terms of a metering pulse signal, the systemsand methods of the present invention are not limited to this particularsignal, and can be applied to other such incompatible signals present ina telephone network, or other network such as a computer network.

Reference is first made to FIG. 1, which is an exemplary system orenvironment in which the present invention may be utilized orimplemented. FIG. 1 is intended to be illustrative of potential systemswhich may utilize the present invention and is not to be construed aslimiting. The system of FIG. 1 illustrates a portable computer 10 havinga PCMCIA compliant slot 12 which is configured to receive a PCMCIAcompliant card 14, which may be a modem, a network interface card, orany other card. The interface 22 of the card 14 is configured todetachably connect with a connector (not shown) inside slot 12.Inserting the card 14 in slot 12 permits the card 14 to be in physicaland electrical communication with computer 10.

The card 14 also includes a connector 24, which is illustrated in FIG. 1as an RJ type connector, but may be of any type, including but notlimited to, a 15-pin connector or a coaxial cable connector. Theconnector 24 is configured to removably receive a plug 26, which isconnected to one end of the cable 28. The other end of the cable 28 isconnected to a plug 26′, which is capable of detachably connecting withthe jack 30. The jack 30 is typically connected to a telephone network,a private branch exchange (PBX) system, or any type of computer network.A user needing access to the telephone network or other system gainsaccess to that system through the jack 30. Through the jack 30, orthrough any other system access point, the computer 10, and morespecifically card 14, is capable of communication with the network towhich jack 30 provides access. With regard to the metering pulse signal,the jack 30 provides access to the telephone network and the card 14 iscapable of receiving the signals generated by the telephone networkthrough the jack 30.

Reference is next made to FIG. 2, which is a block diagram of anexemplary system implementing a preferred embodiment of the presentinvention. The telephone network 38 comprises a central office 40, whichis responsible for generating the signals that are pertinent to theoperation of telephone network 38. The central office 40 is the sourceof the dial tone, the ringing signal and other signals; one example is ametering pulse signal, which is produced by the metering signal source42. The signals generated by central office 40 are transmitted over wirepair 45, which comprises tip 44 and ring 46 in this embodiment. Wirepair 45 is an example of a telephone loop through which central office40 applies various dc and ac voltages. Any device connecting totelephone network 38 usually sends and receives signals over the wirepair 45 and through the central office 40.

A modem 50 is an exemplary connecting device that is capable ofinterfacing with telephone network 38. As described previously, themodem 50 is required to comply with the specifications detailed by thetelephone network 38, including any requirements of the applicableregulatory agency. For example, the telephone network 38 may specify acharacteristic impedance Z₀, which is usually on the order of 600 ohms.In order to function properly, modem 50 must have a terminationimpedance of 600 ohms such that signal reflection is minimized and powertransfer is maximized. The termination impedance specified by telephonenetwork 38 is provided in this embodiment by termination circuitry 60,described in further detail below.

FIG. 2 also illustrates how presently preferred embodiments includeattenuation circuitry 70, which is preferably designed to meet at leasttwo objectives. First, attenuation circuitry 70 must comply with therequirements of telephone network 38. Secondly, the attenuationcircuitry 70 prevents the metering pulse signal (or similar incompatiblesignal) from interfering with the operation of the modem 50 or otherconnecting device.

With respect to the first objective, as noted previously the telephonenetwork 38 typically mandates that the metering pulse signal must not beattenuated by more than a certain amount at the terminals of theconnecting device. Presently preferred embodiments of the attenuationcircuitry 70 addresses this objective by providing a circuit path forthe metering pulse signal that is sufficiently isolated from tip 44 andring 46. The isolation of the attenuation circuitry 70 ensures that themetering pulse signal is not attenuated by more than the amountspecified by telephone network 38 at the terminals of modem 50. However,with respect to the receive amplifier 82 of modem 50, the metering pulsesignal is attenuated such that it does not interfere with the operationof modem 50.

Basically, the function of attenuation circuitry 70 is similar to thatof a physical notch filter or low pass filter circuit. A low pass ornotch filter is used in some instances to prevent signals havingparticular frequencies from passing. The attenuation circuitry 70functions like a low pass or notch filter in the sense that only desiredsignals are attenuated in a preferred embodiment, while the data andother audio signals are not affected by the attenuation circuitry 70.Thus the loop action attenuates the signal such that the specificationsof telephone network 38 are satisfied with regard to the attenuation ofthe metering pulse signal at the terminals of the connecting device andthe operation of the connecting device is not impaired. Additionally,the attenuation loop, that includes attenuation circuitry 70, preventsthe metering pulse signal from saturating receive amplifier 82 andprevents the metering pulse signal from interfering with the data beingtransmitted to the modem 50.

FIG. 3 is a more detailed block diagram of a presently preferredembodiment of the termination circuitry 60 and the attenuation circuitry70. Because of the similarities between impedance synthesis circuitry 92and impedance synthesis circuitry 90, the following discussion, whichoften refers to impedance synthesis circuitry 90, also applies toimpedance synthesis circuitry 92.

In this embodiment, the specified impedance that is to be provided bythe termination circuitry 60 is equal to the characteristic impedance ofthe telephone network. Thus, impedance synthesis circuitry 92 intermination circuitry 60 generates an impedance equivalent to thecharacteristic impedance Z_(o) of the telephone network to which modem50 is connected. In a similar fashion, the impedance synthesis circuitry90 in attenuation circuitry 70 generates an impedance that minimizes theadverse effects of the metering pulse signal on modem 50 and the databeing transmitted and received by modem 50. In both instances, thespecifications of telephone network 38 are satisfied.

In the illustrated embodiments, the attenuation circuitry 70 and thetermination circuitry 60 are not subject to the tolerances of physicalcomponents, because they are able to digitally emulate the behavior ofideal circuit components. More specifically, impedance synthesiscircuitry 90 and 92 preferably utilize a programmable digital signalprocessor (DSP) or the like, and software components, to synthesize adesired impedance to attenuate the metering pulse signal or match thecharacteristic impedance. This approach is not subject to the variabletolerances of physical circuit components. Also, the impedancesynthesized is more accurate, especially when the analog to digitalconverters (ADC) and the digital to analog converters (DAC) have highresolution.

With regard to the illustrated embodiment, the impedance (Z_(s)) that isprovided by the impedance synthesis circuitry 92 is substantially equalto the characteristic impedance Z_(o) of the telephone network. Theimpedance provided by the impedance synthesis circuitry 90 is related tothe metering pulse signal characteristics. By way of example, theimpedance synthesis circuitry 90 uses a detection circuit to detect theline voltage (V_(t)) across tip 44 and ring 46. This voltage V_(t) isthen utilized to produce the specified impedance (Z_(s)), by providing acorresponding current value (I_(t)). Thus, the current (I_(t)) presentacross the terminals of the wire pair or across the full wave bridgeterminals is the sensed voltage divided by the specified impedance(I_(t)=V_(t)/Z_(s)). By application of Ohm s law, the impedance seenacross tip 44 and ring 46 is: V_(t)/I=V_(t)/(V_(t)/Z_(s))=Z_(s). In thismanner, impedance synthesis circuitry is capable of generating impedanceacross a pair of terminals or wire pair such as tip 44 and ring 46.

As is illustrated in FIG. 3, the attenuation circuitry 70 preferablyincludes a resistor 75, which has a value that is large in comparison tothe characteristic impedance of telephone network 38. Resistor 75 servesto isolate the attenuation circuitry 70 from the tip 44 and the ring 46leads of the telephone line. Thus, the impedance of attenuationcircuitry 70 has a minimal effect on the synthesized impedance oftermination circuitry 60. In this way, the primary function of resistor75 is to comply with the requirement of the telephone network that themetering pulse signal, as well as other signals, not be attenuated atthe terminals of modem 50 or other connecting device as describedpreviously.

The transmit amplifier 80 causes the Rx and Tx signals to be summedtogether as a form of feedback for the attenuation circuitry 70. Morespecifically, the Tx signals may be separated from the RX signals bysumming the appropriate level of negative Tx signal to the combined Rxand Tx signals, which allows for an Rx signal with a greatly reduced Txcontent at the receive amplifier 82.

Reference is next made to FIG. 4, which is a schematic diagram showingone embodiment with the relevant components for attenuation circuitryand termination circuitry. The telephone communication networkassociated with wire pair 45, comprising tip 44 and ring 46 lines,delivers a metering pulse and data signal to the connector from meteringand signal source 42. The connector must not attenuate the meteringpulse signal by more than 3 dβ on wire pair 45. Additionally, thecharacteristic impedance 41 must be properly terminated on the connectorside for efficient data transfer. In this embodiment a DSP or modem dataprocessor 50 performs the attenuation of the metering signal via notchfilter 70 and the generation of characteristic impedance viacharacteristic impedance synthesis circuitry 60. Some of the functionsof the attenuation circuitry include, but are not limited to, preventingthe metering pulse signal from saturating the receive amplifier 82 andpreventing the metering pulse signal from interfering with the databeing transmitted to the modem 50. The attenuation circuitry 70 isisolated from wire pair 45 by resistor 75, which has a value that islarge in comparison to the characteristic impedance of telephonenetwork. The termination circuitry 60 produces impedance substantiallyequal to the characteristic impedance (Z_(o)) 41 of the telephonenetwork. Resistor 75 prevents the impedance of attenuation circuitry 70from affecting the value of the impedance synthesized by terminationcircuitry 60. As such, in the absence of a metering pulse signal, whichis transmitted over wire pair 45 at determined intervals, theattenuation circuitry 70 is effectively an open circuit compared to theimpedance of the line. The attenuation feedback loop is summed with thetransmit signal of modem 50 at the transmit amplifier 80. The digitallysynthesized output of transmit amplifier 80 is filtered by terminationfilter 60. The output of the termination filter 60 is first converted toa controlling voltage by DAC 64 and then into a controlling current byV-I converter 68, which controls current source 66. The current source66 generates a current I_(t) so as to provide the desired characteristicimpedance across wire pair 45 as described above.

Likewise, when a metering pulse signal is present on the wire pair 45,the data processor receives a line voltage from ADC 58 and produces anoutput voltage that is calculated based upon a desired low pass or notchimpedance (Z_(notch)) needed to sufficiently attenuate the meteringpulse signal. DAC 74 converts the output of notch filter 70 into acontrol voltage, which is converted into a controlling current by V-Iconverter 78, which controls current source 76. Thus the control voltageoutput by DAC 74 controls current source 76. The current generated bycurrent source 76 corresponds to the desired notch or low pass impedancevalue and attenuates the metering signal accordingly.

Thus the attenuation circuitry and termination circuitry configurationdepicted in FIG. 4 creates several advantages. First, many connectiondevices incorporate a DSP, ADC and DAC interfaces, and memory. Thus, thenumber of extra electrical components is reduced, thereby reducingmanufacturing cost and complexity. Second, the attenuation performed bythe processor based attenuation circuitry has much more accuracy ascompared to attenuation circuitry composed of physical circuitcomponents or notch filters. Finally, the necessity of an externalfilter podule and its associated costs are eliminated. While theimpedance synthesis has been generated with a DSP or modem dataprocessor, it is understood that other configurations, including digitalcircuits, filters and other circuit configurations may be used.Additionally, it is understood that the impedances synthesized by theattenuation circuitry and the termination circuitry may be synthesizedusing the same ADC, DSP, and DAC that are frequently already present onthe connecting modem device.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by United States LettersPatent is:
 1. A circuit for isolating and attenuating a selected networksignal transmitted on a communication network having a wire pair toconnect with a connecting device, the circuit comprising: an attenuatingcircuit connected between the connecting device and the wire pair of thecommunication network, wherein the attenuating circuit synthesizes anattenuating impedance having a predetermined magnitude such that theselected network signal is attenuated by a predetermined amount; and aresistive circuit connected to substantially isolate the attenuatingcircuit from the wire pair.
 2. The circuit as in claim 1, wherein theresistive circuit comprises a resistor having a resistive value that isrelatively large in comparison to a characteristic impedance of thecommunication network.
 3. The circuit as in claim 1, wherein theattenuating circuit comprises: an analog to digital converter circuitthat provides a digital representation of the magnitude of a linevoltage potential across the wire pair; a digital signal processingcircuit that provides a control signal having a magnitude that iscalculated based upon the magnitude of the line voltage and a desiredattenuating impedance; and a current source that produces a currenthaving a magnitude that is substantially equal to the line voltagedivided by the desired attenuating impedance.
 4. The circuit as in claim3, wherein the attenuating circuit further comprises a digital to analogconverter circuit that provides a control voltage for the current sourcebased in part on the control signal received from the digital signalprocessing circuits.
 5. The circuit as in claim 1, wherein theattenuating circuit attenuates the signal at an input of a receiveramplifier of the connecting device.
 6. The circuit as in claim 1,further comprising a termination circuit that provides a synthesizedtermination impedance having a magnitude that is substantially equal toa characteristic impedance of the communication network.
 7. The circuitas in claim 1, wherein the network signal is a metering pulse signal. 8.A circuit for attenuating a metering pulse signal generated by atelephone network, the circuit comprising: an attenuating circuit toattenuate the metering pulse signal using an impedance that issynthesized to a predetermined value; and an isolating circuit to allowthe attenuating circuit to receive the metering pulse signal from thetelephone network when the metering pulse signal is present, and toisolate the attenuating circuit from the telephone network when themetering pulse signal is absent.
 9. The circuit as in claim 8, whereinthe isolating circuit comprises a resistor connected across a wire pairof the telephone network.
 10. The circuit as in claim 8, furthercomprising an impedance circuit to provide a termination impedance,wherein the termination impedance is synthesized to a predeterminedvalue that substantially matches a characteristic impedance of thetelephone network.
 11. A circuit for isolating and attenuating aspecified network signal over a wire pair of a telephone network by apredetermined amount, the circuit comprising: a termination circuit tosynthesize a termination impedance having a predetermining value thatsubstantially matches a characteristic impedance of the telephonenetwork; an attenuation circuit to attenuate the specified networksignal by a predetermined amount; and a resistor connected between theattenuation circuit and the wire pair of the telephone network, whereinthe resistor has a value that is relatively large in comparison to thecharacteristic impedance of the telephone network.
 12. The circuit ofclaim 1, wherein the resistive circuit is to substantially isolate theattenuating circuit from the wire pair of the telephone network when theselected network signal is absent on the wire pair.